This invention relates generally to the field of braking control wherein the electrical data processing system or calculating computer is designed to maintain vehicle velocity at a specified value, and in particular, to a system and method for closed loop speed control for stop and go applications.
Cruise control systems have become common on consumer and commercial vehicles, allowing drivers to select a desired speed and maintain the desired speed without the need to hold down the accelerator pedal. Such systems reduce driver stress and allow them to maintain greater attention to the road. Adaptive cruise control systems provide even greater convenience by monitoring the following distance to the vehicle ahead and adjusting the cruise control as the distance changes.
Although such control systems are available for high-speed operation, they are ineffective when substantial braking torque is required. One such situation is stop and go driving at low speeds, where the vehicle speed is often below the engine idle speed. Similarly, they are not effective for certain environmental conditions, such as a long or steep downhill grade.
Prior art speed control strategies provide braking torque using brake by wire, regenerative braking by the engine, or a combination of the two. Typically, the speed control algorithm is proportional control. While this approach may be acceptable for some applications, it does not allow for the stopping and starting of a vehicle in traffic and may result in irregular operation that is uncomfortable for the driver.
Accordingly, it would be desirable to have a system and method for closed loop speed control for stop and go applications that overcomes the disadvantages described.
One aspect of the invention provides a system for closed loop speed control for stop and go applications, comprising a proportional controller, an integral controller, and a summer. The proportional controller calculates a speed control error from the vehicle speed command and the actual vehicle speed and uses the speed control error to increase or decrease the braking torque applied. The integral controller integrates the speed control error after the adaptive cruise control system disables the speed control, increasing the braking torque applied. The summer combines controller outputs into a torque command that is sent to the traction control system.
Another aspect of the invention provides a stopped controller for reducing braking torque applied after the vehicle stops, overcoming the larger static friction.
Another aspect of the invention provides an open loop controller for supplying a feed forward torque command, corresponding to the vehicle speed command below vehicle idle speed.
Another aspect of the invention provides a switch for setting the torque command to zero for large positive speed control errors, indicating the vehicle speed command is above the vehicle idle speed.
Another aspect of the invention is a transition logic controller for ramping the torque command from the value used in an alternate operating mode and the value to be used in the speed control mode when shifting to the speed control mode.
The invention provides the foregoing and other features, and the advantages of the invention will become further apparent from the following detailed description of the presently preferred embodiments, read in conjunction with the accompanying drawings. The detailed description and drawings are merely illustrative of the invention and do not limit the scope of the invention, which is defined by the appended claims and equivalents thereof.